1. Field of the Invention
The present invention relates generally to vapor-compression refrigeration systems and more particularly to refrigeration systems utilizing a liquid pump to increase liquid refrigerant pressure between a condenser and an expansion device and to refrigeration systems having a liquid injection line to reduce superheat in the compressor discharge manifold and outlet stream. The present invention also relates to refrigeration systems utilizing a liquid refrigeration pump in any portion of the refrigeration system or circuit. Further, the present invention relates to a compressor-pump unit in which a liquid-refrigerant pump and a compressor are enclosed within a single, hermetically sealed housing and are coupled to a common shaft driven by a driving device which may also be enclosed within the housing.
2. Description of the Prior Art
In the United States and other countries, refrigeration systems are important for providing cooling in buildings and automobiles and in enabling safe and inexpensive food storage and transportation. The importance and number of refrigeration systems are continuing to grow with further industrialization and urbanization and as the growing population increases the demand for housing, automobiles, refrigerators, and similar products. The main purpose of a refrigeration system is to cool an enclosed space or medium to a lower temperature and to discharge absorbed heat into a higher temperature medium, such as air outside the enclosed space or other medium. To accomplish this type of cooling, it is necessary to do work on a refrigerant, such as ammonia or a halocarbon, to "pump" heat absorbed from the space being cooled into the higher temperature space.
In this regard, the most widely used refrigeration systems are compressor-driven (i.e., vapor-compression) refrigeration systems in which a compressor performs the work on the refrigerant. In typical vapor-compression refrigeration systems, cooling is achieved by passing a refrigerant through the following four basic components: an evaporator, a compressor, a condenser, and an expansion device or a valve. During operation, high pressure liquid refrigerant from the condenser passes through the expansion device, which reduces the pressure and the temperature of the liquid refrigerant. This low pressure. low temperature liquid refrigerant flows through the evaporator and evaporates as the refrigerant absorbs heat from air or liquids passing through or in heat exchange contact with the evaporator. The gaseous refrigerant is then drawn out of the evaporator by the compressor, which pumps the gaseous refrigerant to the condenser by raising the refrigerant pressure, and thus the refrigerant temperature. The gaseous refrigerant condenses to a liquid in the condenser as it gives up heat to a cooling medium that is passed through or in heat exchange contact with the condenser. The liquid refrigerant then flows to the expansion device where the cooling cycle begins again.
The efficiency or coefficient of performance (COP) of the vapor-compression refrigeration cycle can be measured as the ratio of heat absorbed in the lower temperature area to the amount of work that is put into the system, which, for the above system, would be the amount of energy required to operate the compressor.
While effective in providing cooling, a continuing concern with vapor-compression refrigeration systems has been the cost to initially purchase, to maintain, and to operate these refrigeration systems. A key component of the operating costs is the cost of energy for operating or driving the compressor. The cost of energy is generally the cost of electricity, because compressors are often driven by an electric motor, although internal combustion engines, steam turbines, and other driving devices may also be employed. To control or reduce energy costs, it is desirable to maintain and, more preferably, to increase the efficiency of the refrigeration system to obtain a desired amount of cooling at lower energy input levels, i.e., less work performed by the compressor. By increasing the efficiency of the refrigeration system, maintenance costs may also be improved as components, such as the compressor, are operated at conditions and at capacities more closely matching the conditions for which the components of the refrigeration system were designed and selected. With the widespread use of these refrigeration systems, refrigeration components and refrigeration systems having enhanced efficiency would be highly desirable in reducing the operating and maintenance cost of each system as well as resulting in a very large worldwide savings in operating (i.e., energy savings) and maintenance costs.
One method of increasing refrigeration system efficiency is to maintain the cooling levels or heat absorption levels while reducing the amount of work input to the refrigeration system by the compressor and other components. U.S. Pat. No. 4,599,873 issued to Hyde achieved a reduction in compressor work by reducing the required condensing pressure, i.e., the compressor output pressure, by installing a stand alone, liquid pump in the refrigeration system between the condenser outlet and the expansion device. The liquid pump inputs work to the system by boosting the liquid refrigerant pressure from the condenser thereby providing liquid refrigerant with more cooling capacity, i.e., subcooled liquid refrigerant to the expansion device. In the refrigeration industry, this concept has been labeled liquid pressure amplification (LPA) and has resulted, in a limited number of retrofit applications, in substantial energy savings, increased refrigeration capacities, and extended equipment, e.g., the compressor, service life as the compressor work input may be reduced to provide a condensing pressure that may be lower due to the use liquid pressure amplification.
However, the liquid pressure amplification concept as disclosed by Hyde has not been widely accepted by the refrigeration industry for use in either retrofitted or newly installed, private and industrial refrigeration systems. This lack of industry acceptance is due in part to the initial cost of the stand alone, liquid pump, which may double or at least significantly increase the cost of a vapor-compression refrigeration system. The high cost of the stand alone, liquid pump is due in part to the need for a durable unit that is sealable to prevent refrigerant leakage. Hyde discloses a design having a pump driven by a motor with both the pump and the motor being separately sealed in housings to prevent leakage and contamination of the refrigerant stream in the event of a motor failure. While this liquid pressure amplification design effectively reduces energy costs, the air conditioning and refrigeration industry is highly competitive on initial or installation costs and skeptical of non-mainstream technology, which often requires customizing of existing refrigeration systems and support equipment. Therefore, widespread adoption of liquid pressure amplification for new refrigeration system applications and for retrofit of existing refrigeration systems will probably not occur until a lower cost implementation of this energy saving concept is discovered.
Other efforts toward increasing refrigeration system efficiency have been directed toward increasing the efficiency of the condenser. The function of the condenser is to receive higher pressure, higher temperature gaseous refrigerant from the compressor, to condense the gaseous refrigerant, and to output liquid refrigerant. Generally, the compressor outputs gaseous refrigerant that is superheated or, in other words, contains more heat at a given pressure than would be expected of that particular gaseous refrigerant if the refrigerant was saturated vapor. Therefore, the first portion of the condenser, for example the first 30 percent, must be utilized to remove this extra heat, i.e., to desuperheat the refrigerant vapor to obtain saturated vapor at a given pressure, prior to removing the heat necessary to condense the refrigerant to liquid. To compensate, condensers with large or excess capacity are often employed to condense the superheated refrigerant vapor, thereby adding to the cost of the refrigeration systems.
In an attempt to resolve this inefficiency, U.S. Pat. No. 5,664,425 issued to Hyde discloses a refrigeration system employing liquid pressure amplification (LPA) but designed to try to reduce the temperature of the refrigerant vapor prior to the condenser inlet. This system includes a branch conduit from the stand alone liquid pump discharge line to divert liquid refrigerant into the inlet pipe of the condenser. The lower temperature liquid refrigerant acts to cool or remove heat from the refrigerant vapor before the refrigerant vapor enters the condenser. In this manner, the condenser receives the refrigerant vapor at a lower temperature at which the refrigerant vapor may or may not be desuperheated to saturation, and the condenser's efficacy is increased as more of the condenser volume may be utilized in condensing the refrigerant vapor.
However, the bypass-conduit system disclosed by Hyde has several limitations which have limited its implementation in vapor-compression refrigeration systems. For example, this Hyde system is designed for installation in existing systems after the completion of extensive, and often expensive, analysis of the particular system's operating parameters, including the specific refrigerant being used and the condenser inlet temperatures and pressures. The amount of liquid refrigerant to be diverted may then be calculated from this and other system specific data, and the control of the volume of diverted liquid refrigerant is achieved by selecting a fixed orifice and/or diameter of the bypass conduit. While Hyde's bypass-conduit system has the potential of increasing the efficiency of the analyzed and retrofitted system, the sizing of a bypass conduit for the millions of existing refrigeration systems may not be practical and may make the system only suitable for retrofitting high operating cost refrigeration systems for which the high costs of individualized analysis, design, and customization of the system may be economically justifiable. Further, a fixed-size bypass conduit does not accommodate changing system pressures and temperatures as is desirable in existing, as well as yet to be built, refrigeration units that operate in a wide range of outdoor temperatures and cooling load conditions.
Consequently, in spite of the above discussed efforts to improve vapor-compression refrigeration system efficiency, there is still a need for refrigeration system methods and apparatus which improve the operating efficiency of refrigeration systems employing a wide variety of refrigerants and equipment, such as compressors and condensers, at an acceptable initial cost and with a technical design that is acceptable to the refrigeration industry, i.e., technology that is perceived as mainstream for the refrigeration industry and that is readily useful in typical refrigeration applications.